What do you think I should do?

So a while ago i suspected my dad not being as faithful to my mom as i thought he was. I didn't have hard evidence but i know for a fact he goes on tumblr and comments on girls nudes and stuff. I also once saw someone sent a message on his phone saying i love you so much or something.

But what really broke me today was when we were all in the car, he went down to pick up some stuff and left his phone, my mom sitting in the passengers seat next to him sees a bunch of messages like "you seem like you're in love with her", you can't stop thinking about her and stuff like that. It wouldn't make sense to say something like that about someones wife, my mom, of 20+ years, right?he's also very very protective of his phone. He never lets us look at anything on his phone. My mom and I travelled for a few weeks out the country so that also makes me wonder if something might have happened. And so after my mom saw those messages and she pretended everything was okay and normal but i saw her face go in shock, especially how she kept reading it over, and over, and over in disbelief. I really wanna confront her about it and ask if she's okay but she doesn't know I know and I don't wanna cause problems. I'm so lost right now, I'm crying. What exactly should I do?